This invention is related to catalysts for fuel cells. More, particularly, this invention is related to tungsten-containing compounds which are useable as fuel cell catalysts.
Fuel cells produce electricity by converting reactants such as hydrogen, hydrocarbons, and oxygen into products such as water and carbon dioxide. In general, a typical low-temperature fuel cell comprises an anode and cathode separated by an electrolyte. The anode and cathode consist of a conductive support having a thin layer of a catalyst uniformly dispersed over the surface of the support. During operation, a continuous flow of fuel is fed to the anode while, simultaneously, a continuous flow of oxidant is supplied to the cathode.
In a conventional low temperature H2/O2 fuel cell, a hydrogen gas fuel is oxidized with the aid of a platinum catalyst at the anode to generate electrons and protons which travel by separate paths to the cathode. The electrons are conducted through an external circuit and the protons are conducted through the electrolyte. At the cathode, oxygen gas combines with the electrons and protons to produce water, again with the aid of a platinum catalyst. The current generated by the electrons flowing through the external circuit can be used for work.
A fuel cell configuration of particular importance is the proton-exchange membrane (PEM) fuel cell. In a typical PEM fuel cell, the electrolyte is a solid polymeric material capable of conducting protons, e.g., a perfluorosulfonic acid polymer (e.g., Nafion(copyright) by DuPont). The proton-conducting polymer membrane is sandwiched between membrane electrode assemblies (MEA) formed from platinum catalysts dispersed on carbon black. Examples of these devices are described in U.S. Pat. Nos. 6,030,718, 6,040,007 and 5,945,231 which are incorporated herein by is reference.
Platinum catalysts are preferred for fuel cells because of their high electrochemical activity. However, platinum is expensive and easily poisoned by the trace amounts of carbon monoxide typically found in hydrogen fuels. Numerous attempts have been made to find less expensive electrocatalysts or reduce the sensitivity of platinum catalysts to carbon monoxide. Several of these attempts have focused on tungsten and molybdenum compounds, and in particular their carbides and oxides. For example, U.S. Pat. No. 5,922,488 describes a CO-tolerant anode catalyst which uses a carbon-supported, platinum-dispersed, non-stoichiometric stoichiometric hydrogen tungsten bronze having the formula Pt-HxWO3 wherein x ranges from about 0.05 to about 0.36. U.S. Pat. No. 5,298,343 describes a polycomponent electrocatalyst comprised preferably of platinum or palladium and a chemical component selected from the group consisting of tungstic acid, molybdic acid, ammonium tungstate, ammonium molybdate, sodium tungstate and sodium molybdate. U.S. Pat. No. 5,945,231 contemplates combining tungsten carbide with ruthenium oxide or ruthenium to form a catalysts for a direct liquid-feed fuel cell. Unfortunately, these tungsten and molybdenum-based catalysts have not been shown to exhibit an acceptable level of electrochemical activity for practical fuel cell application without the additional presence of an expensive co-catalyst. Therefore, it would be an advantage to have a tungsten-containing fuel cell catalyst which exhibits a high electrochemical activity without an expensive co-catalyst.
It is an object of the invention to obviate the disadvantages of the prior art.
It is another object of the invention to provide a tungsten-containing fuel cell catalyst which has an electrochemical activity which approaches that of platinum.
It is yet another object of the invention to provide a method of making a tungsten-containing fuel cell catalyst in situ in a fuel cell.
It is still another object of the invention to provide a method for regenerating a tungsten-containing fuel cell catalyst in situ in a fuel cell.
In accordance with one object the invention, there is provided a tungsten-containing fuel cell catalyst comprising a carbon support having a surface layer containing tungsten, the tungsten-containing catalyst generating a power output which is greater than about 20 percent of a power output of an equivalently prepared platinum catalyst when operating under the same conditions using an electrochemical oxidation of hydrogen.
In accordance with another object of the invention, there is provided a method of making a tungsten-containing fuel cell catalyst comprising the steps of:
(a) depositing a tungsten-containing precursor on a carbon support;
(b) contacting the support with a solution of an electrolyte;
(c) applying a voltage to the support sufficient to form the tungsten-containing catalyst.
In accordance with yet another object of the invention, there is provided a method of regenerating a tungsten-containing fuel cell catalyst comprising the steps of:
a) contacting the catalyst with a fluid-based carbon source;
b) applying a voltage to the catalyst sufficient to substantially regenerate the catalyst.